Formatting for citation drop-down in mobile VisualEditor

Per mock-up at
https://trello.com/c/xCo6uLAZ/9-5-references-dropdown-from-toolbar-styling

There are still 2 items in the acceptance criteria that are not included
in this patch, but they are waiting on assistance from the VE team. See
bugs 65377 and 65374

Change-Id: I855c5c83705b80ea5bd66be5b42e28710c87a2ba
Merge "Formatting for citation drop-down in mobile VisualEditor"